1. Field of the Invention
The present invention relates to an enclosure for electric device, of the type comprising an outer envelope made of molded, electrically insulating material. The present invention relates more specifically but not exclusively to a cylindrical enclosure of this type, for use in a surge arrester.
In the present description and in the appended claims, the expressions such as "insulating" and "insulation" are related to electric insulation between electrically conducting pieces under voltage or grounded.
2. Brief Description of the Prior Art
A surge arrester is an electric device which is connected in parallel with another electric apparatus, in order to protect the latter apparatus against overvoltages produced between the terminals thereof. The insulation level of the electric apparatus and consequently the manufacturing costs thereof can therefore be reduced when a surge arrester is used in combination with such an electric apparatus. More specifically, a surge arrester is normally an open circuit which becomes a closed circuit parallel to the electric apparatus to be protected when a significant overvoltage appears between the terminals of the apparatus.
The surge arresters presently available on the market, which are utilized in networks for the transmission or distribution of electric energy, comprise in most of the cases an envelope of porcelain having the general aspect of a cylindrical tube sometimes closed at one end, and a pile of disk-like varistors mounted within the envelope of porcelain. As it is well known, varistors are electrically active elements made of metal oxide or of silicium carbide, and whose impedance varies non linearly when subjected to an overvoltage so as to provide for adequate overvoltage protection. Upon occurrence of a fault in a surge arrester, the varistors are permanently short-circuited whereby an electric arc is produced inside the envelope, which electric arc generates high, explosive pressures as well as temperatures overstepping the melting point of all the known metals. In the prior art, pressure limiting mechanisms have been designed to protect the envelopes of the surge arresters against explosion caused by an internal short-circuit. These pressure limiting mechanisms transfer the electric arc from the inside to the outside of the envelope by means of diaphragms and by means of nozzles orienting the hot gases, so as to eliminate the high, internal pressures.
However, such pressure limiting mechanisms are expensive, due to the necessity of mounting the same on an envelope of porcelain. Indeed, the envelopes of porcelain exclusively used up to now in the construction of surge arresters cannot tolerate, at moderate costs, the mechanical tensions required by such pressure limiting mechanisms. For that reason, these mechanisms are mostly provided in surge arresters installed in high voltage stations of networks used in the transmission of electric energy, the unitary price of such surge arresters being presently ten times higher than that of the surge arresters installed in networks for the distribution of electric energy, operating at voltages lower than 35 kV, and provided with no pressure limiting mechanism.
Accordingly, the surge arresters presently installed in networks distributing electric energy being provided with no pressure limiting mechanism, they are susceptible of explosion upon occurrence of an internal, high pressure. However, their cost remains lower than that which would result from the increase in insulation level of the electric apparatus to be protected. If such surge arresters are rendered non explosive by providing them with pressure limiting mechanisms, their cost, as mentioned hereinabove, is multiplied by ten, and consequently installation of conventional, non-explosive surge arresters in networks used in the distribution of electric energy is not economically advantageous.
Another drawback of the existing surge arresters installed in distribution networks is that, in most of the cases, they are mechanically supported through a metallic band encircling their envelope of porcelain nearby the center thereof, which metallic band being attached to a mechanical support structure often electrically grounded. This type of support requires an exaggerated extension of the envelope of porcelain in the axial direction so as to increase the distance between each of the two electric ends of the surge arrester and the metallic support band whereby adequate insulation between the metallic support band and each of the two electric ends of the surge arrester is obtained. Of course, this type of construction contributes in increasing the cost of the surge arresters.
A further drawback of the conventional surge arresters used in networks distributing electric energy is their lack of humidity tightness. Of course, the anchors adapted to the porcelain and capable of withstanding high mechanical tensions, which are used in the non explosive surge arresters of the high voltage stations, are prevented from being used to increase the pressure applied on the gaskets, because of their prohibitive cost.
Recently, numerous synthetic insulating materials formed with aggregates and binders including epoxy, polymers or other substances, have produced dielectric characteristics comparable to those of porcelain. Moreover, these synthetic materials have two incontestable advantages over the porcelain, namely its capacity to withstand very high mechanical tensions close to the tensions concrete can withstand, as well as the possibility of molding it on pieces of metal or of other materials.